That's a stat for Federal prisons - it's a completely different set of laws and crimes than state and local laws, and only 10% Americans incarcerated are in federal prisons.

Here's a breakdown by most serious offense for my state's prison system:

https://www.doc.sc.gov/research/InmatePopulationStats/ASOF-M...

Which has 16% drugs, but also:

26% homicide, 12% Robbery, 11% Burglary, 9.5% Rape/Sexual assault, 6.6% Kidnapping, 6.4% Assault.

I wrote an blog post a while back about how prison populations skew so much towards the most violent crimes.

http://braino.org/essays/why_so_many_people_in_prison/